sh: Delete unnecessary checks before the function call "mempool_destroy"

The mempool_destroy() function tests whether its argument is NULL
and then returns immediately. Thus the test around the calls is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Rich Felker <dalias@libc.org>